tools: enable no-redeclare rule for linter

PR-URL: #5047
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.eslintrc

@@ -50,6 +50,8 @@ rules:
   # list: https://github.com/eslint/eslint/tree/master/docs/rules#best-practices
   ## require falls through comment on switch-case
   no-fallthrough: 2
+  ## disallow declaring the same variable more than once
+  no-redeclare: 2
 
   # Stylistic Issues
   # list: https://github.com/eslint/eslint/tree/master/docs/rules#stylistic-issues